Techniques for arranging solder balls and forming bumps

ABSTRACT

A mask having a plurality of through holes and a mold having a plurality of cavities are provided, and the through holes and the cavities are aligned. Conductive balls are dispensed into the aligned through holes and cavities. Substantially one ball is dispensed into each aligned through hole and cavity, and the mask with the holes and the cavities in the mold are configured and dimensioned such that the balls are substantially flush with, or recessed below, an outer surface of the mask. The mask is removed, the conductive balls are aligned with pads of a semiconductor device, and the conductive balls are transferred to the pads by fluxless reflow in a formic acid environment. Vibrational, electrostatic, and direct transfer aspects are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/121,236, filed May 15, 2008, the complete disclosure of which isexpressly incorporated by reference herein in its entirety for allpurposes.

FIELD OF THE INVENTION

The present invention generally relates to the electrical and electronicarts and, more particularly, to techniques for forming electricalinterconnections.

BACKGROUND OF THE INVENTION

There are many techniques that can be used for the fine pitch solderbumping process for so-called “flip chip” technology, including, forexample, evaporation, electroplating, screen printing, ball drop, C4NP(Controlled Collapse Chip Connection—New Process), and so on.

As smaller solder bump interconnects, at finer pitch, are in greatdemand for flip chip technology, the ball drop method and C4NP, whicheliminate the volume reduction problem, and which have solder alloyflexibility, have recently gained visibility and attention in theindustry. Around 50% volume reduction between solder pastes and finalsolder bumps after reflow makes it difficult to apply the screenprinting method for fine pitch applications. The electric currentinduced composition control can not handle wide alloy range inelectroplating method. The ball drop method and C4NP allow finer pitchand a larger number of pins than the screen printing method, and theyalso allow more freedom in selecting the composition of solder bumps, incomparison to the electroplating method. Note that “pitch” refers to thedistance between the centers of adjacent solder balls.

Since C4NP uses a solder transfer method from a glass mold to a siliconwafer after injecting and solidification of molten solder in thecavities of a glass mold, when considering the use of high meltingtemperature solders such as 97 weight % Pb—3 weight % Sn and 80 weight %Au—20 weight % Sn, there currently does not exist a suitable materialfor sealing the contact between an injection molded solder (IMS) headand a glass mold, because the temperature of the molten solder is toohigh when using high temperature solders.

Inoue et al., in U.S. Pat. No. 6,213,386, disclose a method of formingbumps. Solder balls and a tool having a large number of through-holesare used, and under the condition that the through-holes of the tool arealigned with the pads of the semiconductor device, the solder balls arecharged into the through-holes, pressed to be fixed on the pads, andthen reflowed to form bumps.

Inoue et al., in U.S. Pat. No. 6,460,755, disclose a bump forming methodand an apparatus therefor. In particular, it is stated that a solderbump forming method and an apparatus therefor achieve high reliability,and an electronic part, produced by this method and this apparatus, isalso disclosed. For each of the step of arraying solder balls, the stepof supplying a flux, and the step of mounting the solder balls on aboard, it is checked whether or not any solder ball is omitted, and theprocess is conducted while confirming the condition of the operation,thereby enhancing the reliability and also preventing defective productsfrom being produced.

Shimokawa et al., in U.S. Pat. No. 6,571,007, disclose a ball-arrangingsubstrate for forming bump, ball-arranging head, ball-arranging device,and ball-arranging method. In particular, a ball-arranging substratecomprising a substrate with a main surface, a plurality ofball-arranging holes formed on the main surface for sucking and holdingminute electroconductive balls at the locations corresponding to thoseof electrodes formed on a semiconductor device or a printed circuitboard, wherein when light illuminates the ball-arranging surface toallow optical recognition of the arrangement of the minuteelectroconductive balls by means of the light reflected by the minuteelectroconductive balls and by the main surface, the wave length of thelight of the light source is set in the range of 300 to 900 nm, and thereflectivity is made not more than 50% based on the light source. Areflective mirror should be provided on the rear surface of thesubstrate opposite to the light source, in the case when the substrateis transparent to the irradiated light.

Bolde, in U.S. Patent No. 6,745,450, discloses a method for loadingsolder balls in a mold. Solder balls are loaded into a reservoir havingmultiple exit ports. A removable mold is fitted into the apparatus andthe reservoir is passed across the top of the mold while solder ballsare fed into cavities in the mold. After the reservoir has advancedacross the mold and the mold cavities are filled with solder balls, thereservoir is reset as a roller is simultaneously guided across the moldto seat the solder balls firmly within the mold. Alternatively, theroller may be applied to the solder balls while the reservoir advancesacross the mold, or both as the reservoir is advanced and when it isreturned to its original position.

Takahashi et al., in U.S. Pat. No. 5,976,965, disclose a method forarranging minute metallic balls. In particular, a method for arrangingmetallic balls to form an array of bump electrodes comprises the stepsof immersing a silicon template in ethanol and dropping metallic ballsthrough the ethanol onto the template to receive the metallic balls inthe holes of the template. The metallic balls are free from cohesioncaused by electrostatic charge or moisture. The template may be inclinedin the ethanol. The holes are formed by anisotropic etching a siliconplate.

Kuramoto et al., in U.S. Pat. No. 6,919,634, disclose a solder ballassembly, a method for its manufacture, and a method of forming solderbumps. In particular, a solder ball assembly includes a mask havingfirst and second sides and a plurality of holes formed therein. Eachhole has a first end opening onto the first side of the mask and asecond end. A plurality of solder balls are disposed in the holes, and afixing agent secures the solder balls in the holes. A protective sheetmay be attached to one or both sides of the mask to cover the ends ofthe holes.

Kirby et al., in U.S. Pat. No. 5,431,332, disclose a method andapparatus for solder sphere placement using an air knife. A station in amanufacturing line for the accurate placement of solder balls on a ballgrid array package and for the removal of excess solder balls comprisesa substrate having an array of solder pads, and an adhesion layer on thesolder pads. The station further comprises a stencil placed on top ofthe substrate and having a height between ¼ times the diameter of one ofthe solder balls and 5/4 times the diameter of one of the balls, thestencil having an array of apertures corresponding to the array ofsolder pads and substantially exposing each of the solder pads of thearray, a pallet for holding and transporting the substrate to thestencil and further along the manufacturing line, a dispenser forpouring solder balls in bulk over the stencil, a vibration devicecoupled to the station for urging the solder balls into the apertures ofthe stencil and onto the adhesion layer above the solder pads, and amoving directed column of air across the surface of the stencil toremove excess solder balls from the stencil.

U.S. Pat. No. 5,985,694 of Cho discloses a semiconductor die bumpingmethod utilizing vacuum stencil. U.S. Pat. No. 5,284,287 of Wilson etal. discloses a method for attaching conductive balls to a substrate.U.S. Pat. No. 5,540,377 of Ito discloses a solder ball placementmachine.

SUMMARY OF THE INVENTION

Principles of the present invention provide techniques for arrangingsolder balls and forming bumps. An exemplary method includes the stepsof providing a mask having a plurality of through holes and a moldhaving a plurality of cavities; aligning the through holes and thecavities; and dispensing conductive balls into the aligned through holesand cavities. Substantially one ball is dispensed into each alignedthrough hole and cavity, and the mask with the holes and the cavities inthe mold are configured and dimensioned such that the balls aresubstantially flush with, or recessed below, an outer surface of themask. The mask is removed, the conductive balls are aligned with pads ofa semiconductor device, and the conductive balls are transferred to thepads by fluxless reflow in a formic acid environment.

In another aspect, another exemplary method includes providing a moldhaving a plurality of cavities; and dispensing conductive balls into thecavities, substantially one ball to each cavity, by tipping andvibrating the mold. The cavities in the mold are configured anddimensioned such that the balls are substantially secured therein duringthe tipping and vibrating. The conductive balls are aligned with pads ofa semiconductor device; and the conductive balls are transferred to thepads by fluxless reflow in a formic acid environment.

In still another aspect, still another exemplary method includesproviding an electrically resistive mold having a plurality of cavities;and dispensing conductive balls into the cavities, substantially oneball to each cavity, by charging the balls to a first polarity and themold to a second polarity different than the first polarity. Anadditional step includes aligning the conductive balls with pads of asemiconductor device, while a further step includes transferring theconductive balls to the pads by fluxless reflow in a formic acidenvironment.

In yet another aspect, yet another exemplary method includes providing amask having a plurality of through holes; and aligning the through holeswith pads of a semiconductor device. The mask and the semiconductordevice have substantially similar coefficients of thermal expansion. Afurther step includes dispensing conductive balls into the holes,substantially one ball to each cavity, the through holes beingconfigured and dimensioned such that the balls are one of substantiallyflush with, and recessed below, an outer surface of the mask. Anadditional step includes transferring the conductive balls to the padsby fluxless reflow in a formic acid environment.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 depict certain issues that may be encountered in the useof a squeeze brush, in the case where a current C4NP mold plate wasemployed directly;

FIGS. 3-5 depict steps in an exemplary method, according to an aspect ofthe invention;

FIGS. 6 and 7 show exemplary alternatives to the steps of FIGS. 3 and 4;

FIGS. 8-11 depict aspects of certain non-limiting exemplary embodiments;

FIGS. 12-14 show steps in an exemplary vibrating mold embodiment,according to another aspect of the invention;

FIGS. 15 and 16 show first and second exemplary electrostatictechniques, according to further aspects of the invention; and

FIGS. 17-19 show steps in an exemplary direct technique, according to aneven further aspect of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Current C4NP techniques require injection of molten solder. As noted,application of such techniques is limited for high-temperature cases,due to inability to find a suitable seal material. Aspects of theinvention provide a method for arraying high melting temperature solderballs and forming solder bumps on a semiconductor device. The directarray of solder balls in the cavities of a mold plate, according to oneor more embodiments of the invention, does not require any sealingmaterials (as in IMS). Accordingly, aspects of the invention providetechniques for placing high melting temperature solder balls into a C4NPmold plate to carry out a fluxless solder joining method.

With reference to FIGS. 1 and 2, a current C4NP mold plate 102 could bedirectly used for the array of solder balls 106; however, to make goodcontact between the solder balls 106 and the input/output (I/O) pads ona semiconductor device after the array of solder balls is formed, thetop surface of solder balls 106 should be over the surface of the moldplate 102. For direct dispensing of conductive balls 106 on the currentmold plate 102, by using a squeeze brush 108 (or even an air knife), thesqueeze brush 108 touches the already positioned balls 106 in thecavities 104 of the mold plate 102 during the dispensing process, asmore than one pass of squeezing or brushing is typically needed for thearray of solder balls 106. The same is true in the case of using an airknife. The undesirable touching is best seen in FIG. 2, which depictsalternate forms of cavities 104 including a glass mold cavity 104A and asilicon (Si) pit cavity 104B. Solder balls 106 can be applied in a drystate, or in some instances, in a liquid medium. A non-limiting exampleof a liquid medium is IPA (Isopropyl alcohol). Any suitable liquid canbe used, in order to assist in keeping the balls from moving around. Theliquid medium may, for example, be a liquid selected from the groupconsisting of a fluxing agent, water, alcohol, and a thermallydegradable soluble polymeric adhesive. In some instances, water oralcohol are employed, rather than fluxing agent or adhesive, to achievea fluxless, no adhesive process.

Accordingly, one or more embodiments of the invention provide techniquesfor arraying solder balls in the cavities of a mold plate for fluxlesssolder bumping processes on a semiconductor device.

Note initially that aspects of the aforementioned C4NP process aredisclosed in U.S. Pat. Nos. 5,244,143; 5,775,569; and 7,332,424, thecomplete disclosures of which are expressly incorporated herein byreference in their entirety for all purposes (although the skilledartisan will already be familiar with same). C4NP employs formic acidduring the solder bumping process on a semiconductor device, to achievea fluxless process. The formic acid is a vapor phase but the flux is aliquid phase. Advantageously, the CTE (Coefficient of Thermal Expansion)of a mold plate is matched to a silicon wafer and the mold holds thesolder in position during the transfer process. C4NP reduces waferdamage from corrosion and contamination, and void formation inside bumpsis limited in C4NP, due to it being a fluxless dry process.

One or more embodiments of the invention provide a fluxless bumpingmethod suitable for solders which have high melting temperature. Thesolder may comprise any solder suitable for flip chip interconnects andits melting temperature may be higher than 250° C., including, forexample, Au—Sn (Sn can be over 20 weight %), Sn—Cu (Cu can be over 1.5weight %), Sn—Ag (Ag can be over 5 weight %), Sn—Sb (Sb can be over 10weight %), Sn—Zn (Zn can be over 30 weight %), Sn—Pb (Pb can be over 90weight %), Sn—Ag—Cu (Sn plus 0 to about 1.5 weight % Cu plus over 5weight % Ag, Sn plus 0 to about 3.9 weight % Ag plus over 1.5 weightpercent Cu), and so on. The solder may have a high melting point (highliquidus temperature) so that C4NP is not suitable because of thelimitations of the sealing material. Aspects of the invention can beextended to techniques for arraying of any conductive balls in theposition of input/output (I/O) pads on the wafer. Furthermore, allvalues given herein are exemplary and non-limiting, and, while believedadvantageous for high-temperature applications, techniques of theinvention could be used for lower-temperature applications as well. Forexample, the solder compositions mentioned in this paragraph merelyillustrate capabilities of one or more embodiments of the invention;techniques of the invention can be employed to different compositions aswell.

Reference should now be had to FIG. 3. In one or more embodiments, athrough hole mask 302 is used to assist the array of solder balls orconductive balls 106 on the cavities 104B of mold plates 102. Mask 302may have frame 306. The total depth of the through holes 304 of the mask302 plus the cavities 104B of the mold plate 102 is almost the same asthe diameter of the conductive balls 106, so as to reduce the chance of,or prevent, touching the already-positioned balls 106 during thedispensing process, and so as to reduce the chance of, or eliminate,filling of more than one ball 106 in a cavity 104B. Balls 106 preferablyare recessed below the surface of mask 302 (or at most, substantiallyflush therewith), but as seen in FIGS. 4 and 7, stand above the surfaceof mold 102 when mask 302 is removed.

As seen in FIG. 4, after the array of conductive spheres 106 is placedin the cavities 104B of the mold plate 102, the through hole mask 302 isremoved and the surface of the solder balls or conductive balls 106extends out over the surface of the mold plate 102, which helps thecontact of solder balls or conductive balls 106 with the I/O pads 314 inthe semiconductor device 312. During the process of transferring thesolder balls or conductive balls 106 from the mold plate 102 to thesemiconductor device 312, formic acid enables employment of a fluxlesssolder-bumping method. The cavities 104B in the mold plate 102 can holdthe position of the solder balls or conductive balls 106 during the moldhandling, so that adhesive, such as flux, is not need in this exemplaryprocess. FIG. 5 shows the finished product, with the balls 106transferred to pads 314 of chip 312.

The process of transferring the solder balls from the mold plate 102 tothe semiconductor device 312 in FIG. 4 involves bonding between thesolder balls 106 and the pads 314. During the transfer process, thesolder balls 106 melt and bond to the pads 314 by forming intermetalliccompounds, which is known as a wetting reaction. However, both thesolder balls 106 and the pads 314 have oxide layers on their surfaces.These oxide layers do not melt at the solder melting temperature.Therefore, formic acid is needed to remove the oxide layers and toshield both the solder balls 106 and the pads 314 against furtheroxidation.

In FIGS. 3 and 4, mask 302 may be, for example, glass, Si, ceramic,metal, or polymer, with through holes 304. Mold 102 may be silicon, withcavities in the form of pits 104B. Alternatively, as shown in FIGS. 6and 7, a glass, Si, ceramic, metal, or polymer mold plate 102 withcavities 104A can be employed. Mask 302 can be similar. The finalproduct can be similar to that shown in FIG. 5.

FIGS. 3-7 thus depict examples of a process flow for fluxless solderbump formation according to aspects of the present invention. Thefluxless solder bump formation according to one or more embodiments ofthe present invention includes an alignment process between a throughhole mask 302 and a mold plate 102, a dispersing of solder balls 106into cavities 104A or 104B, an alignment process between mold plate 102and wafer 312 (which can be done, for example, after lifting off themask 302), and a fluxless reflow process under a formic acidenvironment, resulting in the finished product of FIG. 5.

In the solder ball dispensing process, the solder balls 106 are filledinto the through holes 304 by, for example, one or more of the motion ofa squeeze brush 108, vibration of the mask 312, air flow from the top,and the like. Advantageously, in one or more embodiments, no adhesive(such as flux) is required, since the solder balls 106 are positionedinside the cavities 104A, 104B of the mold 102. The mold plate 102enables retention of the solder balls during mold handling. The masks302 may comprise glass, ceramic, Si, polymer, or any material in whichthe through holes can be made. Each of the holes 304 can have a diametersimilar to that of the solder balls. The diameter of each hole 304should be in the range of about 100% to about 195% of the diameter ofthe solder ball 106. Only one solder ball should be located in one hole.

The mold plates 102 may be made from glass, silicon, or any materialused for injection molded solder molds, and the like. Each of thecavities 104A, 104B can have a diameter similar to that of the solderballs. The top diameter of each cavity 104A, 104B should be in the rangeof about 100% to about 195% of the diameter of the solder ball 106 forthe array of one solder ball in one cavity. Only one solder ball shouldbe located in one cavity.

The sum of the mask 302 thickness and the cavity 104A depth should be inthe range of about 100% to about 195% of the diameter of the solder ball106 for the array of one solder ball in one cavity. Only one solder ballshould be located in one cavity.

The depth of cavity 104A should be in the range of about 30% to about95% of the solder ball 106 for the safe positioning of the solder ballin the cavity and the surface of the solder balls 106 extends out overthe surface of the mold plate 102.

In the case of cavity 104B, the wall angles of the pit should becontrolled for only one solder ball should be located in one cavity andthe surface e of the solder balls 106 extends out over the surface ofthe mold plate 102.

With regard to the preceding discussion, note that the diameter of eachmask hole 304 or mold cavity 104 is related to the requirement that‘only one solder ball should be located in one cavity.’ If the diameterof each mask hole 304 or mold cavity 104 is smaller than 100% of theball diameter, the ball can not be located in the mold cavity. If thediameter of each mask hole 304 or mold cavity 104 is bigger than 200% ofthe ball diameter, more than one ball could be located in the moldcavity or mask hole. Further, the sum of the mask thickness and thecavity depth is related to the requirement that ‘only one solder ballshould be located in one cavity.’ If the sum of the mask thickness andthe cavity depth (not 104B but 104A) is over 200% of the ball diameter,more than one ball could be located in the mold cavity or mask hole. Yetfurther, the depth of the mold cavity is related to the requirement that‘the surface of the solder balls 106 should extend out over the surfaceof the mold plate 102.’

As noted, the combined depth of the holes 304 in mask 302 and thecavities 104A, 104B in molds 102 can be such that the solder balls areapproximately flush with, or slightly recessed below, the surface of themask 302 as seen in FIGS. 3 and 6.

In general terms, one or more techniques, according to aspects of theinvention, can be employed to array any conductive balls so as to enablethe conductive balls to be positioned at the same location as the I/Opads 314 of the semiconductor device 312, for the transfer of conductivespheres 106 from the mold plate 102 to the semiconductor device 312

FIG. 8 shows Si mold 102 with cavities 104B including exemplary detailthereof. When the diameter of the solder ball 106 is “e,” as best seenin FIG. 11, width “b” can be in the range from about e to about 2e, anddimension c, discussed further below with regard to FIG. 11, can be inthe range from about 0 to about d. In a non-limiting example, presentedpurely for illustrative purposes, depth “a” can be about 93 microns,width “b” can be about 130 microns, and dimension “c” can be about 34microns when the diameter of the solder ball “e” is about 100 microns.

In one or more embodiments, the depth of hole “a” in FIG. 11 is notrelated to “d” (mask thickness). In the case of cavity 104B (pitcavity), the solder ball 106 can not touch on the bottom of the cavityand “c” is only dependent on the tapered side wall angle of cavity 104B(pit cavity). However, in the case of 104A (round cavity), the solderball 106 definitely touches on the bottom of the cavity and the sum of“a” and “d” should be in the range from about 0 to about “e.”

FIG. 9 shows an exemplary polymer mask 302 with through holes 304 andframe 306. A polymer film with thickness d of about 2 mils (0.002 inchesor about 50 microns) can be employed, in one or more non-limitingembodiments. Frame 306 can be made of any suitable substantially rigidmaterial, for example, of Invar. The thickness “d” of the polymer film302 can have a range of from about 0.5 mils (0.0005 inches or about 12.5microns) up to about the diameter “e” of the solder ball 106.

FIG. 10 shows mask 302 with holes 304 aligned with cavities 104B in mold102. The alignment process may be carried out by, for example, usingcommercially available tools that use optical sensing of fiducials onmask 302 and mold 102. Note that section lines were included in FIG. 8but are omitted from the other figures to avoid cluttering the drawings.FIG. 11 shows the array of balls 106 in the cavities 104B of mold plate102 as well as in the through holes 304 of mask 302. The tops of theballs 106 are recessed below the surface 303 of mask 302. The dimensionsa, b, c, d cab be as above—note that dimension c is the distance of thetop of ball 106 above the surface of mold 102.

By way of review, an exemplary method, according to an aspect of theinvention, includes the steps of providing a mask 302 having a pluralityof through holes 304 and a mold 102 having a plurality of cavities 104A,104B; aligning the through holes and the cavities; and dispensingconductive balls 106 into the aligned through holes and cavities. Thealignment process may, as noted, be carried out using commerciallyavailable tools that use optical sensing of fiducials on mask 302 andmold 102. Substantially one ball (that is, exactly one ball, or one ballin most cavities with an occasional empty cavity or double-filledcavity, that results in an acceptable product yield) is dispensed intoeach aligned through hole and cavity, and the mask with the holes andthe cavities in the mold are configured and dimensioned such that theballs are substantially flush with, or recessed below, an outer surface303 of the mask 302. The mask is removed, as in FIGS. 4 and 7, and theconductive balls are aligned with pads 314 of a semiconductor device312, and the conductive balls are transferred to the pads by fluxlessreflow in a formic acid environment, resulting in the assembly depictedin FIG. 5. The alignment process may be carried out, for example, usingcommercially available bonding tools that employ optical sensing offiducials on the surfaces to be aligned.

In another aspect, as seen in FIG. 12 a tipping and vibrating plate 102without a through-holes mask can be used to form the array of solderballs or conductive balls in the cavities 1204 of the mold plate 102.The vertical side walls of cavities 1204 in mold plate 102 enable thesolder balls 106 to be arrayed by using tipping and vibrating of moldplate 102. The arraying and handling of the solder balls is easy, andcan be accomplished without the need for flux and vacuum pressure,because the centers of the balls 106 are located below the mold topsurface 1205 (that is, the depth of cavities 1204 is greater than theradius of the balls 106).

FIGS. 12-14 thus depict a tipping and vibrating alignment plate 102without a through hole mask. The solder spheres 106 are filled in thecavities 1204 of the mold plate 102 by tipping and vibrating of the moldplate, as in FIG. 12. The mold plate may be tipped back and forth at anangle of between about 0 and about 45 degrees, with a +/−10 degree anglepreferred. The mold may also be vibrated to ensure an even distributionof the solder balls and prevent ball conglomeration. After cavity fill,the mold is positioned on a semiconductor device 312 with I/O pads 314,as in FIG. 13, and then the solder balls 106 are transferred from themold 102 to the semiconductor device 312 by the reflow process in aformic acid environment, without flux or adhesive. The mold plates 102may comprise glass, silicon, or any material used for injection moldedsolder molds, and the like. Each of cavities 1204 can have dimensionssimilar to those of the solder spheres or conductive balls. The moldcavities are preferably slightly larger (typically by several microns)than the solder balls to insure ball insertion. The side walls of thetop of the cavity are preferably substantially vertical to ensure ballretention prior to solder transfer.

By way of review, an exemplary method, according to another aspect ofthe invention, includes providing a mold 102 having a plurality ofcavities 1204; and dispensing conductive balls 106 into the cavities1204, substantially one ball to each cavity, by tipping and vibratingthe mold 102. The cavities 1204 in the mold are configured anddimensioned such that the balls 106 are substantially secured thereinduring the tipping and vibrating. The conductive balls are aligned withpads 314 of a semiconductor device 312; and the conductive balls 106 aretransferred to the pads by fluxless reflow in a formic acid environment.Preferably, the cavities 1204 have substantially vertical side walls anda depth such that centers of the balls 106 are substantially below anouter surface of the mold 102.

In FIGS. 3-7 and 12-14, the cavities 104A, 104B, 1204 of the mold platecan have a pyramidal shape, vertical sidewalls, or other suitableconfigurations, which help with the retention of solder spheres orconductive spheres 106, depending on the etching methods used to formthe mold. Further, a mold plate reflow may assist with handling prior towafer transfer. After ball placement, the solder filled mold platecavities may be reflowed (temperature elevated above the melting pointof the solder), then cooled. The reflowed solder will exhibit anaffinity for (adhesion to) the mold surface, which will assist with moldplate handling (and prevent accidental ball fall out) prior to wafertransfer.

According to another aspect of the invention, an exemplary electrostaticmethod (which does not require a through hole mask) can be used to formthe array of solder balls or conductive balls 106 in the cavities 1504of the mold plate 102 as shown in FIGS. 15 and 16. The solder spheres orconductive spheres 106 can be self aligned to desired mold locations1504 as the solder spheres or conductive spheres 106 are charged to plus(+) and the cavities of mold plate are charged to minus (−). If desired,the opposite scheme could be used (negative balls and positive mold).

In one or more embodiments, one solder ball mold plate 102 could be anundoped (or lightly doped) silicon or glass wafer that provides a highlyresistive path to charge movement. If it is desirable to further improvethe mold's resistivity, an additional oxide 1506 can be thermally grownon the silicon mold (this can be done quite economically), or depositedon the silicon or glass mold 102 so that the highly resistive(insulating) oxide 1506 separates the mold plate from the second object(ball 106). The mold plate 102 can be charged up in various ways, suchas rubbing with a cloth, or charged via an ion generator. In addition,the mold plate 102 can be selectively discharged by various techniques,such as applying a potential, or through the photoelectric effect—theemission of electrons from a surface (usually metallic) upon exposureto, and absorption of, electromagnetic radiation (such as visible lightand ultraviolet radiation). This would be particularly beneficial in thecase of metallic keep-out zones on the mold 102, as discussed below withregard to FIG. 16.

It is preferable from an electrostatic discharge (ESD) as well aselectromagnetic induced (EMI) noise point of view for surroundingelectronics and chips to keep the mold plate discharged and tied to acommon potential or ground when not holding or attracting solder balls,since in general, this would minimize potential ESD discharge andresulting damage, and minimize electrical noise pickup and radiation.However, if the mold plate is left floating, it may not be ideal for theESD or EMI minimization, but would function when charged to a knownpotential for attracting the solder balls. A further consideration inthe pre-charging time or the post dis-charging time is the quality ofthe insulator, whether if be the glass mold itself, another deposited orgrown thin film such as an oxide layer, and the like. The more ideal theinsulator, that is,, the more resistive the quality of the insulator,the lower the charge leakage or bleed-off rate will be until the mold orthin film is intentionally discharged.

Consider relatively conductive solder balls. To avoid discharging themold plate, and hence, loss of attraction of the (conductive) solderballs to the mold plate, it is necessary to isolate the conductive ballsfrom the conductive portion of the mold plate. In principle, this isaccomplished by placing an insulator between the conductive solder ballsand the mold plate. For example, the mold plate may be of an insulatingmaterial, or contain a thin insulator coating. There are variouseconomical methods of depositing thin, uniform, large area, very goodquality insulating materials such as oxides, nitrides, and the like,using such tools employing thin film deposition methods as chemicalvapor deposition, and so on.

With attention now to FIG. 16, a further refinement, which may beemployed in one or more embodiments of the invention, is the deliberateplacement of keep-out zones 1650 for the solder balls on the mold 102 sothat self assembly of the solder balls 106 to desired mold locations1504 occurs. This self-assembly can be hastened by mechanical aids suchas vibration, brushing, and the like. The keep-out zones 1650 may beformed by first depositing a blanket and conductive metal, spinning onan ultraviolet (UV) sensitive photoresist, UV exposure, and developing,followed by metal etching, and photoresist removal. The remaining metalin the keep-out zone 1650 would be charged to the same polarity as themetal solder balls 106 (in this example, both are positive, but in otherembodiments, both could be negative). The magnitude of the repulsiveforce between the same-polarity surfaces is directly proportional to themagnitude of the charges on the keep-out zones and the solder balls, andinversely propositional to the square of the distance between them.

By way of review, still another exemplary method includes providing anelectrically resistive mold 102 having a plurality of cavities 1504; anddispensing conductive balls 106 into the cavities, substantially oneball to each cavity, by charging the balls to a first polarity and themold to a second polarity different than the first polarity. Anadditional step includes aligning the conductive balls with pads of asemiconductor device (similar to what was depicted in FIG. 13), while afurther step includes transferring the conductive balls to the pads byfluxless reflow in a formic acid environment (to obtain a result similarto FIG. 14). Optionally, the mold has keep-out zones 1650 between thecavities 1504, and an additional step includes charging the keep-outzones 1650 to repel the balls 106.

According to still another aspect, in one or more embodiments of theinvention (see, for example, FIGS. 17-19), the mask 1702 hassubstantially the same CTE as the wafer. Substantially the same meansidentical, or sufficiently close to allow mask 1702 to function asdescribed with regard to FIGS. 17-19. For example, Si or C4NP glassmasks can be used to directly arrange an array of solder balls 106 onthe I/O pads 314 in the wafer 312, as shown in FIGS. 17-19. The thoughhole mask 1702 holds the position of solder balls 106 during thefluxless reflow process, and the mask is removed after the formation ofsolder bumps (bumps refer to solder balls attached to the wafer contacts314) on the pad I/Os 314 in the wafer 312. The depth of through holes1704 are almost the same as the diameter of solder balls 106, so as toreduce or prevent the squeeze brush touching the already-positionedballs 106, and to reduce or prevent more than one solder ball 106adjacent a single I/O 314 on the wafer 312. The masks 1702 may beformed, for example, from glass, ceramic, Si, polymer, or any materialin which through holes 1704 can be made and which has a CTEsubstantially similar to that of the wafer 312. CTE matching is ofinterest because when the wafer and mask are heated during solderreflow, a differential thermal expansion between the parts would movethe solder balls off the Bottom Level Metal (BLM) pads, which would beundesirable. For example if the BLM/ball pitch is 400 microns, then thedifferential thermal expansion between parts during reflow should beless than about 10 to about 50% of the BLM pad/ball pitch.

Thus, FIGS. 17-19 depict direct formation of an array of solder balls106 on the I/O pads 314 on the semiconductor device 312, followed bysolder bump formation. In particular, fluxless solder bump formation canproceed as follows. First, an alignment process can be carried outbetween the through holes 1704 of the mask 1702 and the I/O pads 314 onthe wafer. Next, the solder balls 106 are filled into the through holes1704. In the solder ball dispensing process, one or more of the motionof the squeeze brush 108, vibration of the mask 1702, air flow from thetop (in general, from a location outward of the mask, mold and/or balls,depending on the embodiment), and the like, can be used. After thecompletion of the dispersing of solder balls into the through holes,solder balls are reflowed in a formic acid environment, without flux oradhesive, as in FIG. 18. Note that FIG. 18 appears similar to FIG. 17because the formic acid in FIG. 18 is in the vapor phase and notamenable to illustration. After the fluxless reflow process, the mask ispulled up from the wafer, resulting in the finished product in FIG. 19.The masks may comprise glass, silicon, or any material which has asimilar CTE with the wafer (the wafer is typically Si). Once again, byway of review, yet another exemplary method includes providing a mask1702 having a plurality of through holes 1704; and aligning the throughholes with pads 314 of a semiconductor device 312. The mask and thesemiconductor device have substantially similar coefficients of thermalexpansion. A further step includes dispensing conductive balls 106 intothe holes, substantially one ball to each hole, the through holes 1704being configured and dimensioned such that the balls are substantiallyflush with, or recessed below, an outer surface 1705 of the mask 1702.An additional step includes transferring the conductive balls to thepads by fluxless reflow in a formic acid environment, in FIG. 18,resulting in the structure of FIG. 19.

One or more embodiments may advantageously include one or more of thefollowing features and benefits: no need for concern with viscosity,proper thickness and type of flux to keep solder balls stuck in placebefore wafer reflow; application even to very fine pitch applications(under 100 μm); no need for vacuum pressure; no need for concern withfluid mechanic issues as balls drop through a liquid medium, as in someprior art approaches; no need for polymeric fixing agent and protectivesheet for solving the handling issue after arranging solder balls; andno need for adhesion via flux or other material, such as solder paste orconductive adhesive.

The methods described above can be used in the fabrication and packagingof integrated circuit chips; in particular, techniques set forth hereincan be used to make arrays of solder balls for attachment to anintegrated circuit chip. The chip design can be created, for example, ina graphical computer programming language, and stored in a computerstorage medium (such as a disk, tape, physical hard drive, or virtualhard drive such as in a storage access network). If the designer doesnot fabricate chips or the photolithographic masks used to fabricatechips, the designer may transmit the resulting design by physical means(e.g., by providing a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design can then be converted into anappropriate format such as, for example, Graphic Design System II(GDSII), for the fabrication of photolithographic masks, which typicallyinclude multiple copies of the chip design in question that are to beformed on a wafer. The photolithographic masks can be utilized to defineareas of the wafer (and/or the layers thereon) to be etched or otherwiseprocessed.

Resulting integrated circuit chips can be distributed by the fabricatorin raw wafer form (that is, as a single wafer that has multipleunpackaged chips), as a bare die or in a packaged form. In the lattercase, the chip can be mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a mother board or otherhigher level carrier) or in a multi-chip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip may then be integrated withother chips, discrete circuit elements and/or other signal processingdevices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product thatincludes integrated circuit chips, ranging from toys and other low-endor consumer electronic applications to advanced computer products,having a display, a keyboard or other input device, and a centralprocessor. The techniques set for the herein can be used forinterconnecting the chip on chips or chip stacks for 3D applications,chips on wafers, chips on package or package on package.

It will be appreciated and should be understood that the exemplaryembodiments of the invention described above can be implemented in anumber of different fashions. Given the teachings of the inventionprovided herein, one of ordinary skill in the related art will be ableto contemplate other implementations of the invention.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

1. A method comprising the steps of: providing an electrically resistivemold having a plurality of cavities; dispensing conductive balls intosaid cavities, substantially one ball to each cavity, by charging saidballs to a first polarity and said mold to a second polarity differentthan said first polarity; aligning said conductive balls with pads of asemiconductor device; and transferring said conductive balls to saidpads by fluxless reflow in a formic acid environment.
 2. The method ofclaim 1, wherein said conductive balls comprise solder balls with aliquidus temperature of at least about 250 degrees Centigrade.
 3. Themethod of claim 2, wherein said mold is formed of one of silicon andglass.
 4. The method of claim 3, wherein said mold further comprises aninsulating oxide layer.
 5. The method of claim 2, wherein said chargingof said mold comprises rubbing with a cloth.
 6. The method of claim 2,wherein said charging of said mold is carried out with an ion generator.7. The method of claim 1, wherein said mold further comprises keep-outzones between said cavities, further comprising the additional step ofcharging said keep-out zones to repel said balls.
 8. The method ofclaim, further comprising the additional step of keeping said moldsubstantially discharged and tied to one of a common potential andground when not holding or attracting said solder balls.